Drill bits are traditionally formed from a rod/shaft of high strength metallic material by grinding two or more helical gulleys, known as flutes, into the side wall of the rod extending from the operative front end of the rod towards the rear end, leaving a cylindrical shank at the rear end of the rod. The flutes are separated by lands that define the full diameter of the rod. To reduce the drag that would otherwise be experienced between the lands and the wall of the hole being drilled, the trailing region of each of the lands is ground, providing a slightly reduced diameter over this portion of the drill bit, known as a land relief. This leaves only a leading portion, known as a margin of the land, defining the full diameter of the drill. The leading edge of the margin defines a sharp secondary cutting edge with the trailing side wall of the adjacent flute, which is known as a cutting lip. During drilling operations, only the margin portion of the land engages the wall of the hole in the material being drilled, thereby reducing drag acting on the drill bit and, accordingly, reducing the likelihood of the drill bit binding.
The cutting end part of the drill bit is traditionally formed by grinding the end region of the rod to provide a generally conical end part, known as a point, with end or tip faces extending from each land towards either a chiselled edge, for designs with two flutes, or a sharp point tip for designs with three or more flutes. A primary cutting edge is defined by the junction between the leading edge of each of the tip faces and the adjacent trailing side wall of the adjacent flute. It is these primary cutting edges that cut material being drilled at the end of the drill hole. The shavings of swarf cut from the material pass along the flutes towards the rear of the drill bit, thereby creating room for more material to be cut or shaved and passed into and along the flutes for ejection from the rear end of the flutes.
In the body part of the drill bit, the margin forms an included angle with the trailing side wall of the adjacent flute. The smaller this included angle, the sharper the secondary cutting edge is. A sharper cutting edge has traditionally been desired to increase cutting efficiency. A sharper secondary cutting edge provides more aggressive engagement of the material being cut. If the included angle at the secondary cutting edge is too small, this can lead to decreased operator control, undesirably higher torque and uncontrollable cutting power.
When small included angles are provided at the secondary cutting edges, should the operator move the drill bit off-centre during the drilling process, the secondary cutting edges will have a tendency to widen the hole, as the relatively sharp secondary cutting edges will continue to cut and widen the drill hole. This will have an adverse effect on the security of any screw subsequently screwed into the drill hole. In orthopaedic applications this can lead to screw pullout and implant failure.
In orthopaedic applications, sharp secondary cutting edges can also result in potential damage to soft tissue such as tendons, ligaments, adjacent tissue and other vital organs. Flute designs in traditional drill bits tend to engage soft tissue, resulting in the tissue being wrapped around the drill bit, leading to considerable tissue trauma. This can lead to increased trauma to the patient and, possibly, in the case of arterial damage, can lead to death.
In an effort to avoid these problems, greater included angles are typically employed at the secondary cutting edges by controlling the design of the flute, and particularly the cross-sectional radius of the flute. The larger the radius of the flute, the greater the included angle at the cutting edge, resulting in a less aggressive secondary cutting edge. Larger radius flute cross-sections, however, have a tendency to produce a larger drill bit core diameter and decrease the amount of material in the drill bit towards the full overall diameter of the drill bit, thereby reducing the moment of inertia of the drill bit. This results in the drill bit being more prone to destructive failure when a bending or polar moment is applied to the drill bit. Providing a larger flute radius to soften the secondary cutting edge also results in the primary cutting edge, on the cutting end part, also becoming less aggressive, thereby reducing the cutting efficiency of the drill bit.
In orthopaedic applications, most drilling procedures require the drilling of the bone through the centre or hollow part of the bone known as the medullary canal. Drilling to fixate a fracture requires drilling from one side of cortex to the other. These cortices are known as the near cortex and far cortex. Beyond the far cortex lies soft tissues such as muscles, veins and arteries.
Also in some cases bone structures being drilled into generally comprise a hard, dense, thin external layer of compact or cortical bone and an inner layer of lighter, spongy or cancellous bone. The hardness and density of the cortical bone results in it being significantly tougher to drill through than the cancellous bone.
With typical orthopaedic drill bits, it is difficult to judge when the cutting end part is about to break through the cortical bone. This breakthrough occurs almost immediately after the drill bit has progressed through the bone to an extent where the rear end of the primary cutting edge (at the full diameter of the drill bit) first engages the bone surface, providing a hole in the bone surface that is the full diameter of the drill bit.
Once the drill bit breaks through the near cortex it travels through the hollow part of the bone into the far cortex where it breaks through into soft tissue. The soft tissue provides little or no resistance and the axial load applied to the drill bit by the operator, advancing the drill bit, can result in the breakthrough being sudden, with the drill bit rapidly overshooting deep into the muscles, veins and arteries beyond the required hole depth, potentially resulting in significant increased trauma and in some cases, where arterial damage may be caused, death.
Orthopaedic drill bits which have sharp secondary cutting edges may also cause difficulty when drill guides are used to accurately place the drill bit and drill a hole into bone tissue prior to accurate placement of a screw implant. This practice of utilising drill guides has become commonplace in modern orthopaedic surgery. The sharp secondary cutting edge of the drill bit tends to scratch and burr the inside of the guide, leading to potential for the drill bit to become jammed inside the drill guide. The burrs may also break free from the guide and enter the body of the patient. Jamming of a drill bit also prevents subsequent use of the guide after drill bit removal to deploy the implant screw down the guide for accurate bone fixation. The potential for scratching or burring of the guide is also enhanced where a land relief is ground into the trailing region of each land. This reduces the area over which the drill bit contacts the guide if any non-axial force is applied to the drill bit, such that the non-axial force is transferred by contact to the guide over a smaller area, and thus with greater pressure, increasing the chance of scratching or burring of the guide. Sharp, secondary cutting edges, also tear surgical gloves, which can lead to cross-contamination